In recent years, a discharge lamp operation apparatus for dimming or controlling a light has been popularly used in various scenes for presentation and energy saving. For an example of this kind of discharge lamp operation apparatus for dimming a light, generally known is one equipped with a chopper circuit for rectifying a voltage from a commercial power supply, and converting and outputting a desired DC voltage, an inverter circuit for converting DC voltage of the chopper circuit to an AC rectangular wave form voltage, a load circuit equipped with a discharge lamp and LC resonance circuit connected to the inverter circuit. In this kind of discharge lamp operation apparatus for dimming, it is general practice to control a driving frequency of switching elements which is included in the inverter circuit in order to control a light.
However, recently, for this kind of discharge lamp operation apparatus for dimming, in order to further increase dramatic effects, a function to enable continuous and stable dimming even to a dimming ratio as low as several % or less (a ratio of light intensity when dimmed to the maximum light intensity) is required, together with a function to prevent unpleasant flash from being generated when the discharge lamp in the dimming state at such a low dimming ratio is turned off and then turned on again.
For the discharge lamp operation apparatus with dimming stabilized by carrying out dimming of the discharge lamp by controlling the output DC voltage of the chopper circuit that supplies power to the inverter circuit without positively controlling the driving frequency of the inverter circuit, there is a prior art disclosed, for example, in U.S. Pat. No. 5,493,182.
Similarly, for the discharge lamp operation apparatus which dims the discharge lamp by controlling the output voltage of the chopper circuit, there is one disclosed, for example, in Japanese Patent Laid-Open Publication No. 58-204496. Because this discharge lamp operation apparatus has a self-oscillating type inverter circuit, dimming of the discharge lamp changes the inverter operating frequency, making it difficult to operate the inverter circuit constantly at the optimum operating frequency. In addition, for other discharge lamp operation apparatus, there is a device disclosed, for example, in Japanese Patent Laid-Open Publication No. 03-115938. This is an active oscillating type inverter circuit in which the inverter operating frequency is set to the vicinity of a resonance frequency of an LC resonance circuit. By controlling the output voltage of the chopper circuit in this condition, dimming of the discharge lamp is carried out. Setting the inverter operating frequency to the vicinity of resonance frequency of the LC resonance circuit in this way can maintain the discharge lamp voltage to a high level even during dimming, and can avoid unstable discharge of the discharge lamp. However, in either system, the dimming ratio that can achieve stable dimming is still high and it has been impossible to start a discharge lamp at a low dimming ratio without generating unpleasant flash.
In addition, for the discharge lamp operation apparatus, which has a function to prevent unpleasant flash from being generated when the lamp is turned on again, there is a prior art disclosed, for example, in U.S. Pat. No. 5,502,423. In this prior art, as shown in FIG. 1 of the prior-art document, a means for intermittently applying a pulse voltage enough to start the discharge lamp is added in a discharge lamp operation apparatus which includes a chopper circuit for converting AC power supply to DC power, an inverter circuit for converting DC power to high-frequency power, and a load circuit including the discharge lamp connected to the output end of the inverter circuit and is configured to operate the inverter circuit in advance and then, drive the chopper circuit.
In this discharge lamp operation apparatus for dimming, as shown in FIG. 2 of the prior-art document, operation of the chopper circuit is stopped in the preheating period of the discharge lamp, and the peak AC line voltage is entered in the input of the chopper circuit. When the preheating period is finished and the startup period begins, operation of the chopper circuit begins and during the startup period the output voltage of the chopper circuit is kept constant. During this startup period, a pulse voltage is superimposed to the discharge lamp, and this superimposed manner of the pulse voltage is to gradually increase the peak of the pulse voltage as the startup time elapses. This can be achieved by gradually increasing the variation range of the inverter driving frequency. When this kind of control is carried out, even the moment when the discharge lamp moves from the startup state to dimming deeply a light state, generation of unpleasant flash of the discharge lamp can be prevented. Furthermore, in U.S. Pat. No. 5,502,423, the driving frequency of the inverter circuit is controlled as shown in FIG. 17 of the prior-art document so that the discharge lamp does not generate flash even during the deep dimming of 0.5% or lower dimming ratio. That is, by turning ON the transistor Q5 while the transistor Q3 is turned ON, the transistor Q3 is forcibly turned OFF to change the ON period of the transistor Q3. As the ON period of transistors Q2 and Q3 becomes imbalanced by this, the oscillation frequency varies, and the output of the inverter circuit 12 is varied extensively. Thus, it is enabled to start dimming with no flash generated even at the deep dimming where the dimming ratio is as 0.5% or lower.
In the above conventional example, dimming of the discharge lamp is achieved by varying the output voltage of the chopper circuit, that is, the input voltage of the inverter circuit. This method is able to increase the stability of the discharge lamp at the time of dimming as compared to the method of dimming the discharge lamp by varying the driving frequency of the inverter circuit. However, even if any of the method is used, conventionally, the lower limit of the dimming ratio that can achieve stable dimmed lighting was about 3%.
The reason will be described hereinafter.
First of all, with respect to the method of varying the operating frequency of the inverter, FIG. 22 shows characteristics (a) of the discharge lamp LA, and the output characteristics (b) of the discharge lamp operation apparatus when the operating frequency f of the inverter circuit is designated as a parameter, where the intersection point of characteristics (a) of the discharge lamp LA and output characteristics (b) of discharge lamp operation apparatus is an operation stabilization point of the discharge lamp. In FIG. 22, increasing the operating frequency f, that is, lowering the dimming ratio gives rise to a case in which a plurality of intersections of the output characteristics (a) of the discharge lamp operation apparatus and characteristics (b) of the discharge lamp LA exist (for example, points A, B, and C in FIG. 22). In such event, there are cases in which the operation stabilization point instantaneously moves to anywhere in points A, B and C in accordance with the condition of the discharge lamp LA and a phenomenon of unstable lighting of the discharge lamp LA occurs (hereinafter called this phenomenon “a jump phenomenon”). In particular, when this jump phenomenon occurs at the deep dimming operation with the dimming ratio is nearly 3% or lower, the discharge lamp causes going-out. Consequently, only increasing the operating frequency f cannot accommodate deep dimming where the dimming ratio is nearly 3% or lower.
Next, with respect to the method of varying the output voltage of the chopper circuit, that is, the input voltage of the inverter circuit, FIG. 23 shows characteristics (a) of discharge lamp LA and output characteristics (b) of discharge lamp operation apparatus in which the output voltage Vdc of the chopper circuit is designated as a parameter. In FIG. 23, when the output voltage Vdc of the chopper circuit is lowered, that is, the dimming ratio is lowered, unlike the case in which the operating frequency f is increased, the operation stabilization point is constantly set to one point only no matter what value the DC voltage Vdc would take. Consequently, when the output voltage Vdc of the chopper circuit is designated as a parameter, no jump phenomenon occurs in a discharge lamp. However, even in this method, when the dimming ratio reaches nearly about 3% or lower, the discharge lamp LA causes flickering, going-out, etc., and it sometimes becomes difficult to stably turn on the discharge lamp LA. Consequently, these discharge lamp operation apparatuses could not meet dramatic applications where dimming at nearly 3% or lower dimming ratio is required.
In addition, where a pulse startup voltage is applied to a discharge lamp in order to prevent unpleasant flash from being generated when the lamp is turned on again, a complicated control circuit is required to gradually increase the variation range of the driving frequency in order to gradually raise the peak pulse voltage, constituting a factor of increasing the cost of the discharge lamp operation apparatus. In addition, when the control of above-mentioned conventional examples is carried out after the discharge lamp moves to the deep dimming lighting condition, the difference between the oscillation frequency in the deep dimming lighting condition and the oscillation frequency when the pulse voltage is generated is excessively large. Thus it becomes sometimes difficult to generate a pulse voltage with a peak value enough to prevent generation of flash in discharge lamp particularly in the deep dimming lighting condition where the dimming ratio is 0.5% or lower.